In an injection head of this type, known from EP 0 834 386 A1, the internal nozzle adjustment device for the radial adjustment of the internal nozzle cross-section and the external nozzle adjustment device for the radial adjustment of the external nozzle cross-section are arranged at the end disposed outside a moulding machine, in other words a corrugator, thus allowing an adjustment to be performed during operation. Such radial nozzle adjustment devices are used to change the cross-section of a nozzle channel leading to the respective nozzle outlet of the internal nozzle or the external nozzle, respectively, via its periphery in such a way that the desired melt streams are distributed across the periphery in order to be guided to the actual respective nozzle outlet. Nozzle adjustment devices of this type, which are arranged relatively far away from the internal nozzle or the external nozzle, respectively, can be used for injection heads and corresponding moulding machines for producing twin-wall pipes having relatively small nominal widths.
From WO 99/50046 A1 and from EP 2 116 352 A1, it is known to provide various cross-section reductions distributed along the periphery of the melt channel by providing radial adjustments or deformations. This leads to considerable sealing problems and, therefore, to the risk of operational malfunctions.
In the case of injection heads for apparatuses for the production of twin-wall pipes having large nominal widths, in particular nominal widths great-er than DN 400, the external radial nozzle centering, in particular of the internal nozzles, have a major disadvantage as the injection heads and the individual components thereof are very heavy, with the result that the entire injection head may have a weight of several tons. This means that heavy injection head components including the viscous thermoplastic plastic melt in the internal melt channel need to be moved relative to each other along a relatively long path from outside, in other words by means of the respective nozzle adjustment device. This is possible only with a very high expenditure of force while requiring a complicated design so it is extremely impractical when producing twin-wall pipes having a large nominal width. This is particularly true for the internal nozzle adjustment device.